Improved tank for hydrocarbon fuels



J. A. MERRHLL ETAL 2,440,365

mrnovnn mm FOR umaocamaou FUELS Filed Mare}: 19, 1943 Jbmas/i' WEI-r/Y/major/77,5 dab/1.

Patented May 4, 1948 IMPROVED TANK FOR HYDBOCARBON FUELS James A.Merrill, Akron,

Cnyahoga Falls, Ohio, Corporation, Wilmington, ml, a corporation ofDelaware and Lorin B. Sebrell,

assisnors to Wins'ioot Application March 19. 1943. Serial No. 479,756

1 Claim. (01. 154-435) This invention relates to improvements in vesselsintended to contain liquids and, more particularly, to tanks orcontainers for hydrocarbon fuels exposed to conditions under which thetanks may be punctured, as, for example, in military airplanes.

In the construction of fuel and oil tanks for airplanes, a difilcultproblem is presented by the necessity for providing a self-sealing linerwhich will automatically close a bullet hole before any great quantityof gasoline or oil has escaped. Heavy structures and thick walls areexcluded by the requirement that weight and volume, for a given quantityof liquid to be carried, must be kept at a minimum.

This problem has been solved with respect to airplane fuels of analiphatic character, such as into the plane wing. Also, the materialmust IOU-octane gasoline, by providing in a tank liner a layer ofunvulcanized rubber or gum which readily seals after the passage of abullet and which is surrounded and protected by a layer of strongermaterial, such as fabric or cyclized rubber. Such a gum layer is furtherprotected on the inner side by a layer or coating ofhydrocarbon-resistant synthetic rubber, such as Buna- N orbutadiene-acrylonitrile rubber.

The advent of aromatic fuels has, however, presented a problem in thatsynthetic rubber alone is not sufiiciently resistant to aromatic fuelsto be satisfactory. Such fuels may contain benzene, toluene, xylene,cymene, and other aromatic constituents and, while these improve theoctane rating of the fuel to such an extent that available horsepowerfrom a given engine is increased and take-offs may be accomplished atgreatly increased speeds, fuel tanks containing them and constructedaccording to practices found satisfactory withfuels of aliphatic natureswell to a considerable extent and thus gain weight. This gain in weightincreases the dead weight of the plane and softens the tank lining sothat it is more susceptible to destruction by projectiles. A materialsuitable for such use as a barrier to the difiusion of containedliquids, such as hydrocarbon liquids containing aromatic constituents,must possess suflicient elasticity that the layer or coating of thematerial will stretch without breaking when the cell or containerexpands due to internal pressure caused by the passage of a projectile.The material must also be flexible so that the barrier will not crackwhen thecell is collapsed and squeezed together for installation. Forexample, cells for aircraft must often be squeezed through acomparatively small manhole or other opening for insertion maintain itsflexibility at temperatures well below zero, as in high-flyingairplanes, so that the material does not shatter under such conditionsupon impact of a projectile. And finally,

the material mustefliciently resist diflusion of the contents of thecell, a dimcult matter where fuels containing aromatic constituents areinvolved.

It has now been found that a layer or coating of the material known asnylon efiectively resists the action of aromatic fuels when placed onthe inner surface of.a tank lining next to the fuel contents. As aresult, no gasoline penetrates to the interior layers of the liner andthus no swelling and softening of the bullet-sealing composition takesplace.

The materials of which nylon is representative fulfill the abovespecifications for a barrier to the liquid contents of a vessel in allrespects, and no other materials are. at present, known which fulfillthem to the same extent. Thus metal foil has been employed since it isnot soluble in gasoline and the like, but these foils have no elasticityand, therefore, break when the ves= sel expands under internal pressure,nor do they have sumcient flexibility to prevent cracking when flexiblefuel cells are collapsed and squeezed together for insertion into narrowspaces. Likewise, cellulose sheets do'not possess the requiredelasticity and flexibility, nor are they reliable at low temperatures.Nylon-type materials afiord the only compositions which have met withpractical success in this field.

Nylon may be described as a high-molecular straight chain polyamideresulting from the condensation of a diamine and a dibasic acid. Thesematerials are described at length in United States Patents No. 2,130,523and 2,130,948. They are there described as synthetic linear polyamidesresulting from the interaction of a polybasic acid and a diamine. Moreparticularly, the substances which may be used for the purposes of thepres- -ent invention as a barrier to aromatic fuels are the syntheticlinear polyamides prepared from dibasic carboxylic acids and theiramide-forming derivatives combined with an organic diamine whose aminonitrogens carry at least one hydrogen atom and are attached to carbonatoms which are, in turn, attached to other atoms by single bonds only,the reactants being selected so that the sum of their radical lengthsexceeds eight.

By radical length is meant the number of toms 1 the chain of theradical, each molecule of diamine and each molecule of carboxylic acidcontributing this much to the unit length of the polyamide. Thus theunit length of the polyamide is made up of the sum of the radicallengths of the amine and oil the polybasic acid. this structural unithaving at least nine chain atoms. In the case of a polyamide derivedfrom glutaric acid and ethylene diamine the structural unit is The unitlength, i. e., the radical length of the polyamide structural unit istherefore nine, the

number of atoms in the chain of each contribut-- ing radical, diamineand dibasic acid, being added together to give the unit length. Thehydrocarbon-resisting material will be made up of a number of thesestructural units, each having a chain length of at least nine, inrecurring fashion. Such a recurring structural unit may be representedby the formula in which X and X are hydrogen or monovalent organicradicals, preferably hydrocarbon, whose atoms adjacent to nitrogen arecarbon atoms joined in turn to other atoms only by single bonds; R is adivalent organic radical, preferably hydrocarbon, whose atoms adjacentto nitrogen are carbon atoms joined in turn to other atoms only bysingle bonds; and R is a divalent acyl radical. In order that the unitlength shall exceed eight, the ingredients should be so selected thatthe sum of the radical lengths of -N(X)-R'--N(X')- and R exceeds eight.

Suitable materials may be made by heating the selected diamine anddibasic acid, preferably in equimolecular amounts, although an ex cessof one or the other reactant may be present, in a closed vessel at atemperature above the melting point of the polyamide. In general, afinal temperature above 180 C., which may be as high as 270 to 290 (3.,will be required, the reaction being brought up to this temperature froma starting temperature above 120 C., and customarily in the neighborhoodof 160 to 180 C. The vessel may be opened or evacuated in the laterstages of the reaction. Inert diluents, such as benzene, may be presentand basic substances may also be added, such as caustic alkalies orcarbonates. A satisfactory material may be prepared by the reaction ofadipic acid and hexamethylene diamine; others are the polymides derivedfrom hexamethylene diamine and glutaric acid, propylene diamine andsebacic acid,

' pentamethylene diamine and dodecamethylene dlcarboxylic acid, andpentamethylene diamine and ethyl sebacate.

The aromatic gasoline barrier may be applied to the fuel tank liner bypressure and heat, or a coating may be applied by painting or spraying.Sufficient adhesion between the polyamide and the natural rubber andsynthetic rubber layers in contact therewith can be obtained without theuse of an added adhesive, if great care is exercised, although forpractical purposes adhesives are employed. For example, adherence of thepolyamide to Buna-N (butadiene-acrylonitrile rubber) may be secured bythe use of a cement of Buna-N in toluene, while adherence of the nylonto the natural rubber sealing layer is brought about by the use of achlorinated rubber cement, toluene being the solvent. It will also beunderstood that the nylon or other p yamide may contain otheringredients such as plasticizers customarily incorporated in thecommercial product, e. g. polyglycerol.

A preferred form of the invention is illustrated in the accompanyingdrawing which presents an enlarged section oi a fuel tank liner brokenaway to show the various layers going to make up the liner.

As indicated therein, the liner may be made up of an outside layer ofcord fabric properly frictioned with oil-resistant synthetic rubber toresist the action of sprayed or spilt gasoline. which is cross-pliedwith a second layer of such fabric irictioned with natural rubber. Nextto this are two layers of partially vulcanized or un vulcanized rubberwith or without reclaimed rubber mixed therewith, and separated by asheet of rayon cord fabric. On the face of the gum layer nearest thefuel is spread a coating of nylon or other synthetic linear polyamide ofthe above description, and this in turn is succeeded by a thin layer ofoil-resistant rubber, such as Buna-N. This thin layer of oil-resistantrubber contacts the gasoline but, while not sufiicient in itself tocompletely exclude the aromatic fuel i rom the self-sealing layers ofpartially vulcanized or unvulcanized rubber, does not take up suflicientfuel for this to be of any consequence. The layer does, however, protectthe nylon from damage during installation and from attrition by thesloshing of fuel in the tank. Thus it is possible airplanes. It can, ofcourse, be modified in various particulars, as by substitution of otherselfsealing materials for the soft rubber and rubber and reclaimedmixtures described, by the addition of more layers of any of thekindsdescribed or by the omission of one or more of certain of these layers.construction of, other types of containers for liquids, particularlyhydrocarbon liquids, such as crash-proof tanks, storage tanks andbladdertype containers. In these, there may be no need forbullet-sealing means, but a flexible bladder which is resistant todiflusion of the liquid either does away with the necessity for a metaltank and its concomitant weight, or makes it possible to store liquidsin a metal vessel which need not be liquid-tight. Thus, a flexible cellof the abovedescribed construction may be inserted in a steel oraluminum tank or may be placed in an airplane bulkhead without regard tothe tightness of the supporting structure. In smaller sizes, even themetal shell may be eliminated, the liquid container consisting merely ofa. bladder of rubber, synthetic rubber or other flexible material havingan inner surface of the above-described material to resist diflusion ofthe liquid, such as aromatic gasoline. In general, then, the presentinvention comprises a combination of a layer or coating of syntheticlinear polyamide of the type described herein with a layer of materialcapable of self-sealing when penetrated by a projectile, such as abullet.

While there has beendescribed above a preferred embodiment of theinvention, it will be apparent to those skilled in the art that variousThe invention is also useful in the teriorly of the self-sealing layerresistive to pene- 10 tration of aromatic hydrocarbon fuels andcomprising a linear condensation polyamid and a layer of oil-resistantsynthetic rubber only partially resistant to aromatic hydrocarbon fuelslocated interiorly or and protecting said insulat- 15 ing layer againstphysicalI dinage.

6 summons mm The following references are of record in the file or thispatent:

UNITED STATES PATENTS Number Name Date Damseletal. Oct. 24, 1933Carothers no. 16, 1937 Caz-others Sept. 20, 1988 Carothers Oct. 8, 1940Carothera Aug. 12, 1941 Gray et a]. June 8, 194'!

